An antibody-drug conjugate (ADC) having a drug with cytotoxicity conjugated to an antibody, whose antigen is expressed on a surface of cancer cells and which also binds to an antigen capable of cellular internalization, and therefore can deliver the drug selectively to cancer cells and is thus expected to cause accumulation of the drug within cancer cells and to kill the cancer cells (see, Non Patent Literatures 1 to 3). As an ADC, Mylotarg (Gemtuzumab ozogamicin (registered trademark)) in which calicheamicin is conjugated to an anti-CD33 antibody is approved as a therapeutic agent for acute myeloid leukemia. Further, Adcetris (Brentuximab vedotin (registered trademark)), in which auristatin E is conjugated to an anti-CD30 antibody, has recently been approved as a therapeutic agent for Hodgkin's lymphoma and anaplastic large cell lymphoma (see, Non Patent Literature 4). The drugs contained in ADCs which have been approved until now target DNA or tubulin.
With regard to an antitumor, low-molecular-weight compounds, camptothecin derivatives, compounds that inhibit topoisomerase I to exhibit an antitumor effect, are known. Among them, an antitumor compound represented by the formula below
(exatecan, chemical name: (1S,9S)-1-amino-9-ethyl-5-fluoro-2,3-dihydro-9-hydroxy-4-methyl-1H,12H-benzo[de]pyrano[3′,4′:6,7]indolizino[1,2-b]quinolin-10,13(9H,15H)-dione) is a water soluble derivative of camptothecin (Patent Literature 1 and 2). Unlike irinotecan currently used in clinical settings, this compound does not require an activation by an enzyme for exerting an antitumor effect. Further, the inhibitory activity on topoisomerase I is higher than SN-38 which is a main pharmaceutically active substance of irinotecan and topotecan also used in clinical settings, and higher in vitro cytocidal activity is obtained for against various cancer cells. In particular, it exhibits the effect against cancer cells which have resistance to SN-38 or the like due to expression of P-glycoprotein. Further, in a human tumor subcutaneously transplanted mouse model, it exhibited a potent antitumor effect, and thus has undergone the clinical studies, but has not been put on the market yet (see, Non Patent Literatures 5 to 10). It remains unclear whether or not exatecan acts effectively as an ADC.
DE-310 is a complex in which exatecan is conjugated to a biodegradable carboxymethyldextran polyalcohol polymer via a GGFG peptide spacer (Patent Literature 3). By converting exatecan into a form of a polymer prodrug, a high blood retention property can be maintained and also a high targetable property to a tumor area is passively increased by utilizing the increased permeability of newly formed blood vessels within tumor and retention property in tumor tissues. With DE-310, through a cleavage of the peptide spacer by enzyme, exatecan and exatecan with glycine connected to an amino group are continuously released as a main active substance. As a result, the pharmacokinetics are improved. DE-310 was found to have higher effectiveness than exatecan administered alone even though the total amount of exatecan contained therein is lower than the case of administration of exatecan alone according to various tumor evaluation models in non-clinical studies. A clinical study was conducted for DE-310, and also effective cases were confirmed, in which a report suggesting that the main active substance accumulates in a tumor than in normal tissues was present, however, there is also a report indicating that the accumulation of DE-310 and the main active substance in a tumor is not much different from the accumulation in normal tissues in humans, and thus no passive targeting is observed in humans (see, Non Patent Literatures 11 to 14). As a result, DE-310 was not also commercialized, and it remains unclear whether or not exatecan effectively acts as a drug directed to such targeting.
As a compound relating to DE-310, a complex in which a structure moiety represented by —NH—(CH2)4—C(═O)— is inserted between -GGFG-spacer and exatecan to form -GGFG-NH—(CH2)4—C(═O)— used as a spacer structure is also known (Patent Literature 4). However, the antitumor effect of said complex is not known at all.
Human TROP2 (TACSTD2: tumor-associated calcium signal transducer 2, GA733-1, EGP-1, M1S1; hereinafter, referred to as hTROP2) is a single-pass transmembrane type 1 cell membrane protein consisting of 323 amino acid residues. While the presence of a cell membrane protein involved in immune resistance, which is common to human trophoblasts and cancer cells (Non Patent Literature 15), has previously been suggested, an antigen molecule recognized by a monoclonal antibody (162-25.3 or 162-46.2) against a cell membrane protein in a human choriocarcinoma cell line was identified and designated as TROP2 as one of the molecules expressed in human trophoblasts (Non Patent Literature 16). This molecule was also found later by other researchers and also designated as a tumor antigen GA733-1 recognized by a mouse monoclonal antibody GA733 (Non Patent Literature 17) obtained by immunization with a gastric cancer cell line or an epithelial glycoprotein (EGP-1; Non Patent Literature 18) recognized by a mouse monoclonal antibody RS7-3G11 obtained by immunization with non-small cell lung cancer cells. In 1995, however, the TROP2 gene was cloned, and all of these molecules were confirmed to be identical molecules (Non Patent Literature 19). The DNA sequence and amino acid sequence of hTROP2 are available on a public database and can be referred to, for example, under Accession Nos. NM_002353 and NP_002344 (NCBI).
The hTROP2 gene constitutes the TACSTD gene family, together with human TROP-1 (EpCAM, EGP-2, TACSTD1) gene having about 50% homology (Non Patent Literature 21). The hTROP2 protein is constituted by a signal sequence consisting of N-terminal 26 amino acid residues, an extracellular domain consisting of 248 amino acid residues, a transmembrane domain consisting of 23 amino acid residues, and an intracellular domain consisting of 26 amino acid residues. The extracellular domain has four N-linked glycosylation sites and is known to have an apparent molecular weight of about 10 kD plus the theoretical calculated value 35 kD (Non Patent Literature 19).
Neither has a physiological ligand of hTROP2 been identified, nor its molecular functions has been revealed so far. hTROP2 was found to transduce calcium signals in tumor cells (Non Patent Literature 20). In addition, hTROP2 is phosphorylated at an intracellular residue serine 303 by protein kinase C, which is a Ca2+-dependent kinase (Non Patent Literature 18), and has a PIP2-binding sequence in the intracellular domain, suggesting signaling functions in tumor cells (Non Patent Literature 22).
In immunohistochemical analysis using clinical samples, hTROP2 was found to be overexpressed in various epithelial cell carcinomas and to be expressed in epithelial cells in limited types of normal tissues at a low expression level as compared with tumor tissues (Non Patent Literatures 23 to 27). Also, the expression of hTROP2 was reported to correlate with the poor prognosis of colorectal cancer (Non Patent Literature 23), gastric cancer (Non Patent Literature 24), pancreatic cancer (Non Patent Literature 25), oral cancer (Non Patent Literature 26), and glioma (Non Patent Literature 27).
From models using colorectal cancer cells, it was further reported that the expression of hTROP2 is involved in scaffold-independent cell growth of tumor cells and tumorigenesis in immunodeficient mice (Non Patent Literature 28).
In response to such information suggesting the association with cancer, a plurality of anti-hTROP2 antibodies have been established so far and studied for their antitumor effects. Among these antibodies, there is disclosed, for example, an unconjugated antibody that exhibits in itself antitumor activity in nude mouse xenograft models (Patent Literatures 5 to 8) as well as an antibody that exhibits antitumor activity as ADC with a cytotoxic drug (Patent Literatures 9 to 12). However, the strength or coverage of their activity is still insufficient, and there are unsatisfied medical needs for hTROP2 as a therapeutic target.